This invention relates to an air-fuel ratio feedback control method for an internal combustion engine, and more particularly, to such air-fuel ratio feedback control method adapted to compensate for a aging change in the output characteristic of an exhaust gas concentration sensor arranged in the exhaust system of the engine.
A commonly employed air-fuel ratio feedback control method for internal combustion engines is described in e.g. Japanese Provisional Patent Publication (Kokai) No. 57-137633. In accordance with this conventional method, a value of exhaust gas concentration (oxygen concentration) sensed by an exhaust gas concentration sensor (hereinafter referred to as the "O.sub.2 sensor"), which is arranged in the exhaust system of the engine, and a predetermined reference value are compared. Based on the results of the comparison, the air-fuel ratio of the mixture supplied to the engine is subjected to feedback control to obtain a stoichiometric mixture ratio that will maximize the conversion efficiency of a three-way catalyst arranged in the engine exhaust system, thereby improving the exhaust emission characteristics.
The O.sub.2 sensor employed in the above system uses a substance such as zirconium oxide as a sensing element. Utilizing the fact that the amount of oxygen ion which permeates the interior of the zirconium oxide varies depending upon the difference between the partial pressure of oxygen in the atmosphere and the partial pressure of oxygen contained in the exhaust gas, the O.sub.2 sensor senses the exhaust gas oxygen concentration by outputting a voltage which varies as a function of the above-mentioned variation in partial pressure difference.
However, it is known that an O.sub.2 sensor of the aforementioned construction has an output characteristic that changes with the passage of time, and particularly the sensor output characteristic deteriorates when a vehicle equipped with the sensor is put through an endurance run. As time passes, therefore, the controlled air-fuel ratio becomes richer in comparison with that exhibited by the vehicle at shipment from the factory, despite the fact that the feedback control of the air-fuel ratio is performed under the same conditions. Engine operating performance (e.g. driveability), fuel consumption and exhaust emission characteristics are adversely affected unless some measures are taken to deal with this aging change in O.sub.2 sensor output characteristic.